Heretofore, in producing thermoplastic films such as cellulose acylate films for use in liquid-crystal image display devices, a solution-casting method has been principally carried out, which comprises dissolving cellulose acylate in a chlorine-containing organic solvent such as dichloromethane, casting it on a substrate, and drying it to form a film. Of chlorine-containing organic solvents, dichloromethane is favorably used since it is a good solvent for cellulose acylate and since it has advantages in that its boiling point is low (about 40° C.) and it may be readily vaporized in the film-forming and drying step in its production process.
On the other hand, recently, from the viewpoint of environmental protection, it has become strongly required to retard release of organic solvents such as typically chlorine-containing organic solvents. Accordingly, various measures are now taken for almost completely preventing release of organic solvents in outdoor air. For example, employed is a method of preventing organic solvent leakage through a more severe closed system, and even if an organic solvent leaks out by any chance in a process of film formation, employed is a method of installing a gas absorption tower to adsorb and treat it before it is released in outdoor air. Further, before discharged, an organic solvent is burnt with flames or is decomposed with electron beams, whereby the organic solvent is not almost discharged out. However, it is still impossible to completely prevent the release of organic solvents, and further improvements are required.
A melt-casting method of producing a film of cellulose acylate has been developed as a film formation method not using an organic method (for example, see JP-A 2000-352620, 2006-63169). These references describe a technique of prolonging the carbon chain of the ester group of cellulose acylate to lower the polymer, thereby facilitating melt-casting film formation of the polymer. Concretely, cellulose acetate is changed into cellulose propionate or the like, thereby facilitating melt-casting film formation of the polymer.
Recently, it has been proposed to use, in place of cellulose acylate, a thermoplastic saturated norbornene-based resin film for a protective layer of a polarizing element, of which the photoelasticity is low, or that is, the birefringence change under external stress applied thereto is small and the optical properties are therefore hardly influenced (for example, see JP-A 2003-232930).
In case where the above-mentioned cellulose acylate film or thermoplastic saturated norbornene-based resin film is produced according to a melt-casting film formation method, the optical properties of the film produced are satisfactory in some degree. However, even though the optical properties of the film are satisfactory in some degree, the surface smoothness thereof in its formation is insufficient, or the thickness accuracy may fluctuate or there may occur a difference in the surface smoothness between the two surfaces of the film owing to the unevenness of the temperature change in the film, and further, there may occur die streaks and lateral unevenness; and for these reasons, it has heretofore been extremely difficult to obtain a film that is excellent both in its optical properties and in its physical properties as a protective film suitable for polarizing plates.
In case where the thickness accuracy and the surface smoothness of a film are unsatisfactory, the length of the film may fluctuate; and when the film is wound up, then its outward appearance may worsen. Further, when the film is used as a protective film for a polarizing element, then there may occur another problem in that there is adhesion unevenness between the film and a polarizing element. On the other hand, when the film is used as a substrate of a retardation plate, then there may occur still another problem in that it may cause retardation unevenness when stretched. Further, in case where the surface smoothness of the film is unsatisfactory and where the film has die streaks or lateral unevenness and when the film is built in a liquid-crystal display device as a protective film of the polarizing element therein, then we have experiences still another problem in that the film causes blurred display and image distortion.
From the above, it is earnestly desired to develop a thermoplastic film having excellent thickness accuracy and surface smoothness in addition to having good optical properties.